1. Field of the Invention
The present invention relates to a thermoelectric system using a semiconductor, and more particularly to a light-emitting device, cooling device and electric power extracting device using the Peltier absorption effect.
With the increasing levels of integration in large-scale integrated circuits and the increasing scale of high-speed high-capacity computing systems, and the like, the amount of power consumed and the heat generated by such devices has increased, and so the cooling devices required for absorbing these large amounts of heat have also grown in size and capacity. Increase in the size of cooling devices impedes compactification, upgrading and the reduction of power consumption in the system overall. Moreover, with a conventional cooling device, if one area is cooled, heat is generated in another area. Therefore, it is necessary to provide heat diffusing apparatus in the area where heat is generated. If a cooling device which does not comprise a heat-generating region could be achieved, then compactification and reduction of power consumption, and the like, could be expected.
With the advance of research into the application of light energy in medical fields, such as physical abrasion in oncology, orthopaedics and dentistry, and industrial fields, such as precision machine engineering, semiconductor manufacture, and the like, demand has evolved for compact high-output table-top laser devices.
In recent years, with rising consumption of electric power, there has been consistent increase in the consumption of fuels used to make electricity, and problems which impact upon human survival in areas ranging from energy crises to environmental destruction, have been increasing, for instance, the depletion of energy resources, global warming due to carbon dioxide gas, and environmental pollution caused by exhaust gases. There is currently a world-wide search for energy sources which will relieve this problem and for new methods of extracting energy.
An ideal energy source is of course clean and inexhaustible in supply, and desirably the apparatus for extracting this energy is compact, but has high capacity and high efficiency.
Solar energy has been investigated as one possible future energy source, but although it is clean and inexhaustible, the power density falling on the earth is a low level of 1 kW/m.sup.2, and the conversion efficiency of devices (solar cells) for extracting this energy is a low 20%. With improvements, it is predicted that 2-3 times the current figure will be possible, but even so, apparatus would have to be large in order to provide sufficient electrical power.
2. Description of the Related Art
FIG. 7 shows a band diagram of a conventional thermoelectric element. This element comprises metals M.sub.1 and M.sub.2 formed respectively contacting either side of a thermoelectric semiconductor S having a Seebeck coefficient of .alpha., resistance r, and thermal conductivity K. At the cold junction, a potential barrier of energy difference .DELTA.E.sub.c is formed between the Fermi level of metal M.sub.1 and the level of the lower end of the conduction band in the n-type thermoelectric semiconductor S, and at the hot junction, a potential barrier of energy difference .DELTA.E.sub.H is formed between the Fermi level of metal M.sub.2 and the level of the lower end of the conduction band in the n-type thermoelectric semiconductor S.
More precisely, a spike-shaped depletion layer is formed on the semiconductor S side of either junction, and since the thickness of the depletion layer is several nm, or so, electrons from metal M.sub.1 pass to the lower end of the conduction band of the thermoelectric semiconductor S due to a tunnel effect. Therefore, it is possible to regard these electrons as facing an effective potential barrier of .DELTA.E.sub.c.
A voltage (.alpha..DELTA.T+rI) is applied between metals M.sub.1 and M.sub.2. .alpha..DELTA.T corresponds to voltage required to obtain a temperature difference of .DELTA.T between the two ends of the thermoelectric semiconductor S. By passing a current I, the electrons in the metal at the cold junction overcome the potential barrier .DELTA.Ec and energy Q.sub.PC (Peltier absorption) is taken from the metal M.sub.1. Therefore, the cold junction is cooled and its temperature falls to Tc.
This Peltier absorption energy Q.sub.PC is approximately equal to (.DELTA.E.sub.c +2kT). Here, k is Boltzmann's constant and T indicates temperature. More specifically, it represents the temperature multiplied by the following Seebeck coefficient .alpha.. EQU .alpha.=(k/q){1n(N.sub.COND /Nd)+2}(V/K)
Here, k is Boltzmann's constant, q is the unit of electricity, N.sub.COND is the effective state density in the conduction band, and Nd represents the donor density.
Electrons having energy Q.sub.PC move through the semiconductor S to the hot junction, and energy Q.sub.PH (Q.sub.PC +.alpha..DELTA.T.sub.s I : Peltier emission) is supplied to metal M.sub.2. Therefore, the hot junction generates heat and the temperature thereof rises to T.sub.H. Taking the quantity of heat absorbed at the cold junction as Q.sub.C, the quantity of heat generated at the hot junction as Q.sub.H, the Peltier absorption energy as Q.sub.PC, the Peltier emission energy as Q.sub.PH, the Peltier absorption coefficient as .PI..sub.c, the Peltier emission coefficient as .PI..sub.p, the Joule heat generated in the thermoelectric semiconductor S as P.sub.r, the temperature at the cold junction as T.sub.c, the temperature at the hot junction as T.sub.H, the ambient temperature as T.sub.R, and the temperature difference between the hot junction and the cold junction as .DELTA.T.sub.s, the relationships between these temperatures, thermal quantities, and the like, are expressed by the following equations. ##EQU1##
In these equations, (1/2) P.sub.r corresponds to the quantity of heat out of the Joule heat generated inside the thermoelectric semiconductor S which flow to the cold junction, and .DELTA.T.sub.s K corresponds to the quantity of heat out of the heat generated at the hot junction which flows to the cold junction. The formula Q.sub.H -Q.sub.C shown in equation (1.3) gives the value of the electric power supplied externally to the element, and generally this is a positive value. This indicates that electric power is consumed in the element and remains in the element in the form of thermal energy.
In a conventional Peltier element, if an electric current continues to pass through the thermal insulating space, the temperature of the whole element will continue to rise, due to the thermal flow from the hot junction to the cold junction, and the generation and accumulation of heat at the hot junction. At the same time, since the temperature at the cold junction also rises, the element will lose its cooling function. Consequently, if this element is used in a cooling device, a heat diffusing device is essential in order to maintain a uniform temperature and suppress temperature rise at the hot junction due to accumulation of heat. Therefore, the overall dimensions and power consumption of the cooling element are increased.